The present invention relates to wheelchairs, and especially to anti-tip wheels used on power wheelchairs.
A conventional mid-wheel drive power wheelchair, such as that illustrated in FIGS. 1 and 2, typically rests on two drive wheels, one on each side, close to a position directly below the center of gravity, and one or more caster wheels at the back. Many such wheelchairs are also provided with one or more anti-tip wheels at the front to prevent the wheelchair from tipping forward and/or to assist it in climbing curbs and other obstacles.
The front anti-tip wheels may be casters that normally rest on the ground, or may be wheels that are normally above ground. The anti-tip wheels may be fixed, resiliently mounted, or connected to the drive wheel suspensions so as to move up and down actively in response to movement of the vehicle. Examples of wheelchair suspension systems incorporating anti-tip wheels are shown in commonly-assigned U.S. Pat. No. 6,129,165 (Schaffner et al.) and U.S. Pat. No. 5,944,131 (Schaffner et al.).
When a wheelchair approaches an obstacle having a generally vertical face, such as a curb, the front anti-tip wheels are intended to ride up and over the obstacle, lifting the front of the wheelchair and assisting the wheelchair in climbing the obstacle. However, if the wheelchair approaches the obstacle at an oblique approach angle α, as illustrated in FIG. 2, conventional anti-tip wheels may tend to slide along the vertical face of the obstacle rather than mounting it. This effect may be accentuated where the anti-tip wheel is a caster. The flatter the angle α at which the wheelchair approaches the obstacle, the more likely the problem is to arise. Further, as the wheelchair continues to approach the obstacle, the sliding action tends to turn the wheelchair so that it is aligned along the obstacle, exacerbating the problem.
It is therefore an object of the invention to provide an obstacle-climbing wheelchair with front anti-tip wheels that are more likely to mount the obstacle, and less likely to slide along it, when the wheelchair approaches the obstacle at an oblique approach angle α.